1. Field of the Invention
The present invention relates to a reactor for preparing a nuclear fuel and particularly to an improved fluidized-bed reactor for preparing uranium trioxide easily in a continuous and efficient manner.
2. Description of the Prior Art
Usually in a nuclear reactor using enriched uranium as a nuclear fuel, uranium is separated from used nuclear fuel after removing fission products and plutonium and is used again for the nuclear fuel. In this case, uranium is separated usually as uranyl nitrate, so a process for preparing uranium trioxide from uranyl nitrate is necessary.
For preparing uranium trioxide, usually a precipitation method or a direct denitration method is used. The present invention relates to the latter method.
The precipitation method comprises adding ammonia to an aqueous solution of uranyl nitrate, precipitating ammonium diuranate, and baking the precipitate to give a powder of uranium trioxide. A benefit of this method is the ease of elimination of impurities in the residual. However, the method has many defects such as difficulty in establishing a precipitation condition, and it is unsafe. Also, the precipitation method generates a large amount of effluent comprising ammonium nitrate. Many difficulties are encounted treating this effluent.
On the other hand, a direct denitration method comprises directly heating an aqueous solution of uranyl nitrate, concentrating it by evaporation of water, and decomposing it to form uranium trioxide powder. The main reactions are believed to be: ##STR1## The direct denitration method is simpler to handle and uses a simpler reactor compared with the precipitation method. The reason for this is the direct denitration method is a heat decomposition reaction, and small amounts of effluent are produced because additives are not used. Although impurities would remain in the powder, they can be eliminated by purifying uranyl nitrate as a raw material.
Further, in the direct denitration method, a fluidized-bed reactor has often been used because of simplicity of the reactor, little amount of effluent, safety on account of little local heating and cooling, uniform quality of uranium trioxide powder, stability of the process due to good mixing, and the like.
However, long-running continuous operation of a fluidized bed reactor has been difficult. Cakes generate in holes for the supply of the aqueous solution of uranyl nitrate. Big particles caused by caking also generate to prevent good fluidity. Further, adopting proper structure to a fluidized-bed and stable operation is difficult because of the limitation of critical safety shape. For the sake of such defects, the efficiency of preparing uranium trioxide is inadequate.
As the result of many experiments, the present inventor developed a new improved fluidized-bed reactor having a rectangular shape to make long and safe running of a reactor possible in preparing uranium trioxide, to give high efficiency and to improve operation.